The bug is due to a stupid make misfeature. The article [1] contains much more
information but in a nutshell, the following code:
a b: c
bla
is equivalent to:
a: c
bla
b: c
bla
This is bad because in parallel runs (make -j typically), "bla" can be run
TWICE and even worse, twice in PARALLEL. Obviously the result will be
completely unexpected. This is a real bummer because on the other hand,
the following code:
%.c %.h: %:in
bla
actually expresses the fact that bla produces two files. For some reasons,
pattern rules work differently from implicit rules.
This commit attempts to fix the problem with lang.h by rewriting (simplified):
lang.c lang.h: lang.in
genlang
as
lang.h: lang.in
genlang
lang.c: lang.h
This works (it correctly expresses the dependency chain and ensures genlang runs
once) but as one drawback: if one manually removes lang.c, then genlang will not
re-run since the second rule does nothing. This is minor drawback since no one
ever removes lang.c manually and "clean" removes lang.h which triggers a rebuild.
[1]: https://www.gnu.org/software/automake/manual/html_node/Multiple-Outputs.html
Change-Id: Ic0bf7c7c203dc599b00fde457946d2316c70474e
13364c5525